The present invention relates to optical copying machines in general, and more particularly to improvements in copying machines wherein the imaging system moves relative to the support for an original, or vice versa, and includes means for exposing successive strip-shaped portions of the original to modulated light in order to image such portions onto a moving photosensitive carrier, e.g., a web of photographic paper. The exposing means of the imaging system includes a mask or diaphragm which shields the remainder of the original from copying light.
When the imaging system of a copying machine includes one or more light sources (e.g., fluorescent lamps) which are in circuit with a source of alternating current, the intensity of copying light varies at a certain frequency whereby the reproduced image and its background often exhibit alternating rib- or strip-shaped regions or streaks of more and less pronounced density which extend at right angles to the direction of movement and all the way between the marginal portions of the carrier. Such undesirable formations are invariably attributable to fluctuations in the intensity of modulated copying light; they are especially pronounced if the number of light modulations during travel of a given portion of the original with respect to the imaging system or vice versa is relatively small. The just described situation will arise when the intensity of copying light is high, when the sensitivity of the carrier is high, when the carrier is a relatively narrow web and/or when the speed of the original with respect to the imaging system or vice versa is high.
It has been found that the development of streaks is attributable to rather pronounced differences between total amounts of light which impinge upon successive portions of the original during movment of the original relative to the imaging system or vice versa. In other words, and depending on the phase of the imaging system at the start of exposure of a given portion of the original to copying light, the timing of starting and final impulses of the imaging system is not identical for different portions or strips of the original.